Deposited papers are any papers that are placed in either the House of Commons or the House of Lords Library by a minister or the Speaker, which have not been presented to the House in any other way (for example an Act, a Command Paper, or a House of Commons paper). The vast majority of deposited papers are placed by ministers in reply to parliamentary questions. In some cases departments may consider that depositing a paper meets the wish of MPs to be informed about a subject without the need for them to formally publish it. MPs other than ministers may not deposit papers.
Further information on this subject can be found from the following links.
List of deposited papers
About Parliament: Deposited papers